Photolithography is a technique employed in semiconductor device fabrication, in which an optical system is used to accurately project an image of a pattern on a reticle onto photoresist coated on the surface of a wafer and expose the photoresist. Accurately reproducing critical-size features in the reticle pattern on the wafer requires exposure dose control.
FIG. 1 shows an exposure system of a conventional step-and-repeat photolithography machine, in which a mercury lamp is used as a light source. The construction of the whole light path mainly includes the mercury lamp 100, an ellipsoidal reflector 101, a reflector 102, a coupling lens group 103, an exposure shutter 104, a variable attenuator 105, a light homogenizer 106, an energy detector 107, a relay lens group 108, a relay reflector 109, a projection objective 110 and an energy spot sensor 111. Reflected successively by the ellipsoidal reflector 101 and the reflector 102, the light from the mercury lamp 100 forms a light cone outside the lamp chamber. The light cone is then coupled by the coupling lens group 103 into the light homogenizer 106. The light homogenizer 106 is a quartz rod with certain specifications, where the light is reflected for multiple times. The light then exits the quartz rod uniformly at an end surface thereof, which is located in an object plane of the relay lens group 108. As a result, a uniform field of illumination with a certain degree of telecentricity and a certain numerical aperture (NA) is formed on an image plane of the relay lens group 108.
FIG. 2 shows a dose control system in the exposure system of the conventional step-and-repeat photolithography machine, which includes a mercury lamp controller 112, the exposure shutter 104, the variable attenuator 105, the energy detector 107, the energy spot sensor 111 and a dose control circuit board 200. However, the following deficiencies have been identified from the practical use of such a dose control system: 1) since the illuminance of the mercury lamp as a light source is not controllable, it has to additionally employ the variable attenuator for producing illuminance levels required in low-dose exposure applications, which, however, on the one hand, increases the complexity and cost of the system, and on the other hand, reduces the reliability of the system because the variable attenuator is a moving part; 2) as the mercury lamp places demanding operational requirements, such as long turn-on and warm-up times and disallowed frequent switching on/off, it has to additionally employ the exposure shutter, which however, on the one hand, increases the complexity and cost of the system, and on the other hand, significantly reduces the reliability of the system because the exposure shutter needs to frequently move; and 3) the mercury lamp is a potential hazard because it is filled with high-pressure mercury vapor, which, when leaked due to misuse, may seriously harm the environment and workers at the site.